1. Field of Invention
This invention relates to plastic molding methods and apparatus and, more particularly, to methods and apparatus for forming one-piece, hollow plastic articles having a blow-molded portion fused to a second prefabricated portion.
2. Description of the Prior Art
In the art of blow molding hollow plastic articles, a tubular parison is extruded from an extrusion head in a semi-soft condition and at an elevated molding temperature. The parison is formed in the part of a closable blow mold which usually comprises a pair of mold halves having an inner surface which conforms to the desired outer surface configuration of the article to be molded. The mold halves are closed on the parison to form an access opening around a blowing pin at one end of the mold and to pinch off and seal the parison at the other end of the mold. Air is admitted to the interior of the parison through the blowing pin, to expand the parison against the mold surface and mold conducts heat from the molded article to cause the plastic to form a solid molded object. The mold halves are then separated and the article is removed from the molding apparatus.
Many blow-molded containers have a reduced diameter threaded neck which is molded around the mold blow pin and held by the mold while the parison is blown to conform to the remainder of the container. In blow molding containers having relatively wide mouth access openings, certain manufacturing dilemmas are presented. If a relatively large diameter parison is selected to mold a container having a wide mouth opening, relatively little radial expansion of the parison is accomplished and folds and imperfect definitions obtain on the outer surface of the molded article. Moreover, a large pinch-off or tear line occurs across the closed end of the container. If on the other hand, a small diameter parison is used for a container having a relatively wide access opening, the parison cannot mold itself to the wide mouth aperture without leaving an annular cap at the opening which must be later machined.
A solution to the foregoing problem is offered in U.S. Pat. No. 3,371,376, wherein a container having a large mouth opening is blow-molded. The parison has a relatively small diameter compared to the diameter of the container and the neck opening to be molded. The patentees provide an expanding device which includes a plurality of fingers designed to move radially apart in the horizontal plane after receiving the parison to stretch the open mouth of the parison to conform to the wide open mouth of the container. During the expanding operation, however, the mold segments are closed on the parison and air must be blown into the parison to support the parison prior to sealing the mold. This technique, however, not only results in an uncontrolled manipulation of the parison but requires a considerable amount of excess compressed air in the molding operation, and therefore increased production costs.
In addition it has been found that containers made by the standard blow molding methods are not suitable for certain applications, as, for example, for holding fluids at very high pressures.